This invention relates to a method of and apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product.
It is known to extract natural gas from underground reservoirs. The natural gas often contains nitrogen. The nitrogen may be in part or totally derived from nitrogen which has been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. A feature of such operations is that the concentration of nitrogen in the natural gas tends to increase with the passage of time from about 5% by volume to about 60% by volume.
U.S. Pat. No. 4,415,345 discloses a process for rejecting the nitrogen from the methane in a double rectification column operating at cryogenic temperatures. A double rectification column comprises a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the top of the higher pressure rectification column in indirect heat exchange relationship with a region, usually the bottom, of the lower pressure rectification column. In the process according to U.S. Pat. No. 4,415,345 a stream of a mixture of nitrogen and methane is cooled at elevated pressure to a temperature suitable for its separation by rectification. A part of the feed gas is liquefied. The resulting gas mixture is separated by rectification in a lower pressure rectification column having a condenser-reboiler operatively associated with a bottom region thereof. The reboiling passages of the condenser-reboiler are heated by pressurised nitrogen. Typically, a part of the nitrogen is separated in a higher pressure rectification column and another part flows in a heat pump circuit to and from the top of the lower pressure rectification column. At low nitrogen levels in the feed gas, the higher pressure rectification column is by-passed and all the nitrogen for the condenser-reboiler flows in the heat pump circuit, which is separate from the main heat exchanger used to cool the feed gas stream. As the nitrogen content in the feed gas gradually increases so the higher pressure column is used to provide some of the nitrogen. Eventually, the heat pump circuit is closed and the higher pressure column produces all the nitrogen.
The higher pressure rectification column used in the process according to U.S. Pat. No. 4,415,345 is operated with a second condenser-reboiler at its bottom. As a consequence, there is a high vapour flow therethrough and it needs to be of a relatively large diameter even though it is operated at maximum capacity only at high nitrogen concentrations.
It is an aim of the present invention to provide a method and apparatus that keeps down the vapour traffic through the higher pressure rectification column and thereby makes it possible to keep down the diameter of this column.
According to the present invention there is provided a method of rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the mole fraction of nitrogen in the feed gas increasing over a period of time. The method comprises cooling the feed gas stream in a main heat exchanger, and rectifying the cooled feed gas stream in a first double rectification column comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, expanding at least part of the feed gas stream into the higher pressure rectification column, partially liquefying the feed gas stream upstream of the first double rectification column, and operating a second double rectification column in parallel with the first double rectification column when the nitrogen mole fraction in the feed gas stream has increased to at least a predetermined value.
The invention also provides apparatus for rejecting nitrogen from a feed gas stream comprising methane and nitrogen so as to form a methane product, the apparatus comprising a main heat exchanger for cooling the feed gas stream, a first double rectification column for rectifying the feed gas stream comprising a higher pressure rectification column, a lower pressure rectification column, and a condenser-reboiler placing the higher pressure rectification column in heat exchange relationship with the lower pressure rectification column, and an expansion device downstream of the main heat exchanger communicating with the higher pressure rectification column, the expansion device being arranged so as, in use, to introduce a part of the feed gas stream into the higher pressure rectification column in liquid state, characterised in that there is a second double rectification column in parallel with the first double rectification column.
As the mole fraction of nitrogen in the feed gas mixture becomes greater with the passage of time, so the flow of product methane becomes less, and so less of the feed gas mixture is liquefied against the reduced flow of the product stream. In consequence, there is a tendency for an ever increasing proportion of the feed gas mixture to enter the higher pressure rectification column in vapour state with the passage of time. Thus, in its normal operating lifetime, which may well exceed ten years or more, and last until the reservoir which is the source of the feed gas mixture is effectively exhausted, the first double rectification column may have to cope with a very wide range of vapour loadings, posing considerable design problems.
The obvious solution to these problems would be to arrange for all the feed gas mixture to enter the higher pressure rectification column in vapour state. Therefore, changes in the composition of the feed gas mixture would not substantially affect the vapour loading of the higher pressure rectification column. However, one disadvantage of such a procedure, is that the vapour loading of the higher pressure rectification column would always be at a maximum.
The method and apparatus according to the invention make it possible, however, to reduce the effective range of vapour loadings that both rectification columns of the first double rectification column have to face during their operating lifetime by providing a second double rectification column in parallel with the first double rectification column, which second double rectification column can be brought into operation at a suitable nitrogen mole fraction. Accordingly, if the vapour loading on either column of the first double rectification column approaches a level at which its performance would become unsatisfactory, the second double rectification column may be used to share the load. Typically, the apparatus according to the present invention may be installed as a complete unit. Alternatively the second double rectification column may be retrofitted to the first double rectification column whenever the latter is approaching an operational stage at which either rectification column is overloaded with vapour. The latter alternative offers the advantage that the initial size, complexity and cost of the apparatus according to the invention can be kept down.
Preferably, the operating pressure of the higher pressure column of the first double rectification column is periodically increased. To this end, the lower pressure column of the first double rectification column preferably has a back pressure regulating valve associated with it which is operable to increase the pressure therein. Increasing the operating pressure of the lower pressure rectification column brings about a concomitant increase in the operating pressure of the higher pressure rectification column. Increasing the operating pressure of the higher pressure rectification column enables it to receive more vapour per unit time for operation at a given constant percentage of flood. In addition, increasing the operating pressure of the higher pressure rectification column tends to increase the proportion of the feed gas mixture that passes out of the expansion device in liquid state. These two factors enable the method and apparatus according to the invention to be operated at higher feed gas nitrogen mole fractions without using the second double rectification column than would otherwise be possible.
Preferably, particularly when the mole fraction of nitrogen in the feed gas mixture is less than 0.15, a first flow of gas is passed as a first recycle gas flow from the lower pressure rectification column of the first double rectification column to the higher pressure rectification column of the first double rectification column. The first recycle gas flow is compressed, is cooled in the main heat exchanger, is liquefied in a condenser-reboiler, and is introduced into the higher pressure rectification column. This counteracts a tendency for the double rectification column to be short of reflux when the mole fraction of nitrogen in the feed gas mixture is relatively low, particularly when it is 0.15 or less.
A product methane stream is preferably withdrawn in liquid state from the lower pressure rectification column of the first double rectification column, is raised in pressure, and is vaporised at least in part in the main heat exchanger. A second recycle gas flow from the lower pressure rectification column is preferably compressed, is cooled in the main heat exchanger, and is introduced at least partly in liquid state into the top of the higher pressure rectification column via a second expansion device. Preferably the second recycle flow is compressed to a higher pressure than the first recycle flow, the higher pressure typically being a supercritical pressure. More preferably, the first and second recycle flows are compressed in the same plural stage compressor, the second recycle flow being taken from a stage of the compressor downstream of the one from which the first recycle flow is taken.
It is, however, possible to use separate compressors for these purposes, or indeed to take all the compressed recycle gas at the same pressure and pass it through the same second expansion device. In this latter arrangement a two phase fluid flow passes out of the second expansion device with the vapour part of the two phase flow constituting the first recycle gas stream and the liquid part of the two phase flow constituting the second recycle flow. All these arrangements make it possible for the temperature-enthalpy profile of the streams being cooled in the main heat exchanger to be kept as close match to the temperature-enthalpy profile of the streams being warmed in the main heat exchanger. As a result the main heat exchanger can be operated at a good thermodynamic efficiency.
The back pressure regulating valve is preferably operably associated with means for changing its setting in response to an increase in the mole fraction of nitrogen in the feed gas mixture. If desired, the control means may make use of an algorithm relating the optimum operating pressure of the lower pressure rectification column to the mole fraction of nitrogen in the feed gas mixture. Alternatively, and more preferably, the pressure regulating valve may be controlled so as to maintain a constant percentage recovery of methane in the product gas.
Preferably, a vent stream is taken from the first recycle gas flow upstream of its compression and is vented from the method and apparatus according to the invention.
Preferably, there is a flow control valve operable to control the size of the first recycle flow.
The pressurised liquid product methane stream is preferably warmed, without being vaporised, in a further heat exchanger upstream of its vaporisation in the main heat exchanger.
Preferably, all the bottom fraction obtained in the higher pressure rectification column of the first double rectification column is withdrawn therefrom and is sent to the lower pressure rectification column. There is therefore no reboiling of this fraction in the higher pressure rectification column.